Why does the support truck weight affect the performance of a TMA?
When an errant vehicle strikes the back of a TMA, it begins to push the support truck forward. Lighter support trucks are pushed further forward and at a higher speed than 5 heavier support trucks. Support truck roll ahead produces two beneficial effects. Firstly, any distance that the truck rolls ahead before the impacting vehicle is brought to rest effectively extends the length of the cushion. Extending the effective length of the cushion significantly increases its energy dissipation capacity. Secondly, as the support truck rolls ahead, it picks up speed. Because an impacting vehicle need only to be slowed to the rolling speed of the support truck, roll ahead also reduces the total amount of energy that must be dissipated during the impact. Thus, lighter support trucks greatly increase the capacity of the attenuator and reduce the total amount of energy that must be absorbed by the cushion. No TMA system other than the TTMA-100 has been able to pass all NCHRP Report 350 crash tests
